Induction generators have certain advantages over synchronous generators. Synchronous generators require separate field windings on the rotor which are excited by a separate DC voltage source, thus making the rotor complicated, asymmetrical and difficult to operate at high speeds. Induction generators do not have field windings, and thus have a more symmetrical structure. Therefore, induction generators are cheaper and more rugged than synchronous generators. Also, induction generators can be operated at higher speeds, due to their symmetrical nature and simple structure.
Such high speed machines permit a small size with a relatively large power output, making them especially desirable for use in vehicles, such as automobiles, trains and planes.
The current in the rotor windings of an induction generator is induced by the rotating stator magnetic field. This rotating stator magnetic field must be maintained by applying reactive electric power to the generator.
In the prior art, this reactive electric power was supplied by adding capacitors to the stator winding terminals in parallel with the load. A tank circuit was thus formed by the added capacitors and the inherent inductance of the induction generator. This tank circuit resonates at a certain frequency, after a small charge is placed on the capacitors.
In this conventional induction generator, a negative resistor is referred across the air gap, and is effectively placed in parallel with a capacitor and an inductor. This negative resistor "pumps" or effectively pushes electrical power into the tank circuit, and thus the capacitance supplies reactive power to the induction generator.
Such an arrangement suffers from undesirable drawbacks. For example, the capacitor and inductor assembly has only one resonant frequency, and thus the induction generator will only operate at one speed. The speed could be changed by using an adjustable capacitor, but this is very difficult and expensive, due to the physical nature and cost of such adjustable capacitors.
Another drawback is that there is effectively no limit on the amount of power that is supplied to the tank circuit by the negative resistor, except for inductor saturation. This can lead to uncontrolled, excessive variation of voltage with load.
Further, when the load is changed, the resonant frequency and the voltage also change, since these quantities are load-dependent. In other words, the parameters load, resonant frequency and voltage are interdependent or "coupled." This makes the prior art induction generator very difficult to control, and this control problem is a major reason why induction generators are not widely used.